Effect of pressure on corrosion of Inconel 625 in supercritical water up to 100 MPa with acids or oxygen

The corrosion behavior of Inconel 625 in supercritical water was investigated under high pressure (30-100 MPa at 400 degrees C) with the addition of acids (5 mmol/kg CH3COOH or 0.5 mmol/kg HNO3) or O-2 (2.5-250 mmol/kg). Ni and Cr ions were the main dissolved metal ions in the effluent. The Ni ion concentration showed no clear dependence on the O-2 concentration or pressure in the experiment with O-2; the concentration was lower than 0.05 ppm. The concentration increased with pressure up to 0.09 ppm and 3 ppm (at 100 MPa) in the experiment with CH3COOH and HNO3, respectively. The Cr ion concentration increased with the O-2 concentration and pressure; the maximum value of the concentration was about 0.5 ppm. The Cr ion concentration was lower than 0.01 ppm in the experiment with CH3COOH, while the concentration was considerably higher in the experiment with HNO3: the concentration increased with pressure up to 0.87 ppm at 100 MPa. The effect of pressure on the corrosion behavior of Inconel 625 was discussed using potential-pH diagrams, metal oxide solubilities, pH, and equilibrium of ionic reactions. Pressure dependence of the metal ion concentrations was analyzed using a model with water density as a parameter; the log-log plots revealed a linear relationship. (C) 2014 Elsevier B.V. All rights reserved.